Handle utf8 character as a fill character in alignment

[dsvi]

If you use unicode symbols as fill character for alignment, you get runtime error
for example, for:
print(u8" {:─<5}─{}\n", i, str);
there supposed to be outputs like this

 0─────2019 March 27 06:32:41                                                                                                                                                 
 1─────2019 March 27 14:43:20                                                                                                                                                 

but there is runtime error:
missing '}' in format string

[vitaut]

Thanks for reporting. This is a known limitation: only single code units can be used in the fill field at the moment.

[vitaut]

format_specs can accommodate multibyte specs as of e4f84ee, but need to hook this to the parsing and formatting logic.